Prior to the advent of modern exercise machines and universal gyms, iron weights, used in combination with bars as bar bells or dumb bells and lifted against gravity, were the most common resistance medium against which muscles were worked for exercise. Such free weight exercise apparatus has many disadvantages. There is a danger of loss of control of the weight due to fatigue or an attempt to lift more weight than the muscles are capable of controlling. Much time is required for changing weights and moving weights and auxiliary equipment in preparation for different exercises. Equipment for an extensive and thorough fitness program constitutes a great number of separate parts, including weights and bars to be organized and stored.
contemporary exercise and universal gym devices continue to use weights made of iron or other heavy material to provide resistance for muscle exercise, but confine the weights to movement along fixed tracks to eliminate dangers of loss of control or dropping of free weights. The weights of these apparatus are connected by chains and levers in various configurations, to exercise members which are engaged and worked by an exercising athlete. These machines may provide changing mechanical advantage through the exercise stroke to maximize exercise benefit. However, such machines suffer from a number of disadvantages. They must be massive, to provide the weight necessary for training advanced athletes and to provide necessary structural strength, and they are complex, because all exercise motions must be translated into up and down movement of the weights along their tracks. This latter consideration generally precludes any single machine from providing a sufficient number of different conditioning exercises for a complete fitness program.
Efforts to reduce the mass and complexity of exercise machines and universal gyms have resulted in a number of apparatus utilizing hydraulic resistance. Generally, these apparatus have two key elements in common; a hydraulic cylinder linked to an exercise member to pump fluid in and out of the cylinder in response to movement of the exercise member, and a means for creating resistive pressure in the cylinder against which the muscles are worked. Despite their large number, all such machines are deficient in one or more respects. Most utilize double-action hydraulic cylinders and, unlike free weights, provide exercising forces which resist movement of the exercise member during both an exercise stroke and a return stroke of an exercise cycle. This "two-way resistance" does not provide the benefits of free weight exercise which provides no resisting force during the return stroke.
Many hydraulic exercise devices of the present art do not provide a sufficient number of exercises for true muscle conditioning program versatility. Many of these machines utilize multiple hydraulic cylinders in an attempt to provide a sufficient number of different exercises, further increasing their mass and complexity. Generally, substantial time and effort is required to change between exercising configurations of these apparatus. None of these devices provide for controlled variation of exercise resistance over the exercise stroke to provide optimum exercise benefit.